Might want to consider a PHP captcha script, yes you'd have to port it into plugin form(or maybe just write a bit of code to intergrate it), but it would open up the possibilities a great deal. This one apparently provides lots of customization: phpcaptcha.org ... you may also wish to check out some of the others listed here: hotscripts.com/category/php/scripts-programs/security-systems/…
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t31osJan 28 '11 at 15:19

Thanks for the "custom" adjective. I hadn't tried that yet.
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JohnDec 26 '10 at 19:43

@John: oh that's cool feedback. I just have learned the last days that site: accepts subdirectories within URLs for google search. But I smell that such a plugin does not exists so far, right? Let me know if there is anything close to your needs at last. I like what you ask for.
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hakreDec 26 '10 at 23:54